Strong gravitational fields are significantly modified by higher curvature corrections. The most well known such corrections are given by the Lovelock class of theories [1]}. This class has a number of noteworthy features of interest, including having second order differential equations and
a particle spectrum that is the same as Einstein gravity. However from a phenomenological perspective they have the disadvantage that they are trivial in
4 spacetime dimensions. Recently two newer classes of higher-curvature gravity have been discovered. One is
Quasi-topological gravity [2]}, [3]}, [4]}, whose formulation is
also in more than 4 dimensions. Another is Generalized Quasi-Topological Gravity (GQTG) [5]}, [6]}, [7]}, which
are constructed by requiring that there is a single independent field equation for only one metric function under the restriction of spherical symmetry. They
have attracted interest because such theories have the same graviton spectrum as general relativity on constant curvature backgrounds and are non-trivial in 4 spacetime dimensions. As such they provide a new set of phenomenological competitors to general relativity in strong-field regimes, whose parameters can be constrained by observation.
